MySQL: Remove collation compatibility check for strings

[alu]

Compatibility check in<str as sqlx::Type<sqlx::MySql>>::compatible(tv) for collation is not necessary because collation does not affect to output string.

RELATE: #1241

[abonander]

I'm trying to remember why we initially discounted BINARY_FLAG and BLOB_FLAG for telling the difference between a text string and a binary string, but nothing's coming to mind.

[alu]

As the title suggests, I tried to remove the judgment by collation from the comatibility method, but since the type could not be judged correctly, I returned to correcting only the field name.
Specifically, this test fails because _1 is judged as a string instead of a byte string.

[alu]

Since it became possible to judge with flags, a new commit was added.

[alu]

@abonander
Hi!

Is it not possible to merge this change?

[abonander]

I merge PRs as I get to them. SQLx is only a small part of my full-time job currently.

[abonander]

My gut is telling me this should wait for a major release as it could change some type mappings in the macros, but I'm not sure on that.