[MIPS] Malta: remove a dead function declaration

Neither is the mips_rtc_get_time() routine defined anywhere in
the MIPS architecture-specific code, nor does anyone call it any
more. This patch removes the extern declaration of this fossil.

This patch does not introduce any functional changes.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>