mesa/main: remove stale prototype

This function was removed a long time ago.

Fixes: 8e581747d2 ("mesa/formats: make format testing a gtest")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29528>

src/mesa/main/formats.h

@@ -730,9 +730,6 @@ _mesa_format_row_stride(mesa_format format, int width);
 extern GLenum
 _mesa_uncompressed_format_to_type(mesa_format format);
 
-extern void
-_mesa_test_formats(void);
-
 extern mesa_format
 _mesa_get_srgb_format_linear(mesa_format format);