MIPS: uprobes: remove set but not used variable 'epc'

Fixes gcc '-Wunused-but-set-variable' warning:

arch/mips/kernel/uprobes.c: In function 'arch_uprobe_pre_xol':
arch/mips/kernel/uprobes.c:115:17: warning: variable 'epc' set but not used [-Wunused-but-set-variable]

It's never used since introduction in
commit 40e084a5 ("MIPS: Add uprobes support.")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: <ralf@linux-mips.org>
Cc: <jhogan@kernel.org>
Cc: <linux-kernel@vger.kernel.org>
Cc: <linux-mips@vger.kernel.org>