[Messaging] Remove extraneous const on FIRMessagingIsAPNSSyncMessage

[andrewheard]

Removed an extraneous const on the BOOL return type of FIRMessagingIsAPNSSyncMessage(...) to resolve a warning surfaced with internal tooling:

return type 'const BOOL' (aka 'const bool') is 'const'-qualified at the top level, which may reduce code readability without improving const correctness

https://clang.llvm.org/extra/clang-tidy/checks/readability/const-return-type.html

#no-changelog

[andrewheard]

The messaging / spm (macos-14, visionOS, Xcode_15.3) job is already failing on main; ignoring.