MIPS: drop asm/const.h

Commit 86f21c96 (mips: Use common _AC macro now.) removed the _AC
definition from const.h. All other macros defined in const.h are not
used anywhere, and there is now no user of this header. Remove this
header.

Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Signed-off-by: Stefan Roese <sr@denx.de>