Config.mk: Remove stray comment

Config.mk: Remove stray comment

This comment isn't about CONFIG_TESTS, but about SEABIOS_DIR that has
been removed.

Originally, the comment was added by 5f82d0858de1 ("tools: support
SeaBIOS. Use by default when upstream qemu is configured."), then
later the SEABIOS_DIR was removed by 14ee3c05f3ef ("Clone and build
Seabios by default") but that comment about the pain was left behind.
The commit that made CONFIG_TESTS painful was 85896a7c4dc7 ("build:
add autoconf to replace custom checks in tools/check").

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>