build: delete/replace clang warning pragmas

build: delete/replace clang warning pragmas

- delete redundant warning suppressions for `-Wformat-nonliteral`.
  This now relies on `CURL_PRINTF()` and it's theoratically possible
  that this macro isn't active but the warning is. We're ignoring this
  as a corner-case here.

- replace two pragmas with code changes to avoid the warnings.

Follow-up to aee4ebe59161d0a5281743f96e7738ad97fe1cd4 #12803
Follow-up to 09230127589eccc7e01c1a7217787ef8e64f3328 #12540
Follow-up to 3829759bd042c03225ae862062560f568ba1a231 #12489

Reviewed-by: Daniel Stenberg
Closes #12812