Remove pragma GCC diagnostic from source files

[davidwendt]

Description

Removes the #pragma GCC diagnostic from several source files. These do not seem to be necessary and could suppress useful compile warnings.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[vyasr]

@robertmaynard these were added in #14726. Has something changed in our compilation since then to make these pragmas unnecessary? Perhaps a change to our compiler toolchain?

[davidwendt]

No compile warnings appear in the CI logs. I tested with a debug build as well.

[karthikeyann]

#14726 (comment)

The attribute hidden was added because the kernel itself was called across TUs. But now we have a wrapper launch_mixed_join<bool> which is instantiated in 2 files, which in turn calls 2 kernel versions (<true>, <false>) separately in its own TU, so the kernel can be static.
This caused 'attribute hidden' to be removed, but the pragma about warnings was never removed.

[davidwendt]

/merge