meson: remove redundant Wall and Wextra flags
Using the "warning_level=2" mesa option both "Wall" and "Wextra" are already set by default.
With this commit we also get rid of the compiler warnings:
meson.build:117: WARNING: Consider using the built-in warning_level option instead of using "-Wall".
meson.build:117: WARNING: Consider using the built-in warning_level option instead of using "-Wextra".
/cc @smcv @jpwhiting