Revert "helper/schema: Make nested Set(s) in List(s) work"

[catsby]

Reverts #7393

[catsby]

re: failing tests mentioned in #7393:

TF_ACC=1 go test ./builtin/providers/fastly -v -run=TestAccFastlyServiceV1_gzips_basic -timeout 120m
=== RUN   TestAccFastlyServiceV1_gzips_basic
--- PASS: TestAccFastlyServiceV1_gzips_basic (35.94s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/fastly 35.953s
Test:

TF_ACC=1 go test ./builtin/providers/aws -v -run=TestAccAWSSecurityGroup_Change -timeout 120m
=== RUN   TestAccAWSSecurityGroup_Change
--- PASS: TestAccAWSSecurityGroup_Change (30.74s)
PASS
ok      github.com/hashicorp/terraform/builtin/providers/aws    30.763s
Test:

[radeksimko]

Merge at will, I will see if I can turn the mentioned failures into more abstract (resource-agnostic) test cases.

[phinze]

LGTM - @catsby lets kick off AWS and Fastly suites manually after this lands.

[catsby]

New AWS run: https://travis-ci.org/hashicorp/terraform/builds/141378794
New Fastly run: https://travis-ci.org/hashicorp/terraform/builds/141378900

[ghost]

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