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