Don't add -Winline for WARNS=6

This warning is very rarely useful (inline is a hint and not mandatory).
This flag results in many warnings being printed when compiling C++
code that uses the standard library with GCC.

This flag was originally added in back in r94332 but the flag is a no-op
in Clang ("This diagnostic flag exists for GCC compatibility, and has no
effect in Clang"). Removing it should make the GCC build output slightly
more readable.

Reviewed By: jrtc27, imp
Differential Revision: https://reviews.freebsd.org/D29235