Remove duplicate preprocessor constant definition for EF_MIPS_ARCH.

The duplicate was introduced in commit 45506bdd. It placed the
constant EF_MIPS_ARCH in a better place, however it did not remove
the original. This patch removes the original occurrence.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>