x/feegrant completeness audit updates

[blushi]

Description

ref: #9115
Address x/feegrant completeness audit updates:

• remove dead code in https://github.com/cosmos/cosmos-sdk/blob/master/x/feegrant/genesis.go#L9-L21 (we're declaring some GenesisState struct while there's already the proto-generated GenesisState type in x/feegrant/types).
• improve test coverage in InitGenesis in case of error
• QueryServer in x/feegrant/keeper/grpc_query.go is not covered by tests in x/feegrant/keeper/keeper_test.go but indirectly in x/feegrant/client/rest/grpc_query_test.go using grpc-gateway REST endpoints. This x/feegrant/client/rest folder must be some leftovers of legacy REST routes from the initial feegrant module implementation but it doesn't make sense to keep it since the module doesn't support legacy REST routes. Those tests should be moved to x/feegrant/keeper/keeper_test.go.
• For the tx commands, in the cobra command Use field, we use granter and grantee as arguments names in one command and in the other one, we use granter_address and grantee_address. It would be good to have consistent naming across all feegrant commands.
• in tx CLI, we are using seconds for time-based expiration from the user, it should be timestamp

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #9177 (d50c361) into master (fc256a3) will increase coverage by 0.13%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master    #9177      +/-   ##
==========================================
+ Coverage   59.72%   59.86%   +0.13%     
==========================================
  Files         595      595              
  Lines       37346    37355       +9     
==========================================
+ Hits        22306    22361      +55     
+ Misses      13059    13011      -48     
- Partials     1981     1983       +2     

Impacted Files	Coverage Δ
x/feegrant/module.go	45.65% <33.33%> (-2.08%)	⬇️
x/feegrant/genesis.go	90.90% <80.00%> (+44.48%)	⬆️
x/feegrant/client/testutil/suite.go	99.15% <100.00%> (+0.01%)	⬆️
x/feegrant/keeper/grpc_query.go	81.39% <0.00%> (+81.39%)	⬆️

[anilcse]

lgtm, just small nits

[atheeshp]

lgtm