Removes index() from bionic.

This function has been removed from POSIX.

Unfortunately, we can't leave #define index(a, b) strchr((a), (b)) in its place
because defining a preprocessor macro for index() breaks a whole lot of code.

Bug: 13935372
Change-Id: Ifda348acde06da61c12e7ee2f8fe6950a3174dd1

libc/arch-x86_64/x86_64.mk

@@ -1,7 +1,6 @@
 # x86_64 specific configs
 
 libc_common_src_files_x86_64 := \
-    bionic/index.cpp \
     bionic/memchr.c \
     bionic/memrchr.c \
     bionic/strchr.cpp \