Remove _flush_cache(2) for mips64.

Also remove declaration.

The only user is compiler-rt, and they can replace that call with one to
syscall(2). compiler-rt doesn't currently build on mips64 anyway.

Bug: 11156955
Change-Id: Ieae0ba49c8e7aa50253401fc1d7c2d17bc867d39
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index 38ae831..bfa13b7 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -315,7 +315,7 @@
 int     cacheflush:__ARM_NR_cacheflush(long start, long end, long flags)  arm
 
 # MIPS-specific
-int     _flush_cache:cacheflush(char* addr, const int nbytes, const int op) mips,mips64
+int     _flush_cache:cacheflush(char* addr, const int nbytes, const int op) mips
 int     __set_tls:set_thread_area(void*) mips,mips64
 
 # x86-specific