Bug 211368

Bitmap::operator[] never worked.  There's currently no way to use it to read a bit value.  We also can't use it to set a bit value because it relies on Bitmap::iterator::operator= to set the value.  However, Bitmap::iterator stores the Bitmap* as a const pointer, and Bitmap::iterator::operator= calls set() on the const pointer.  If we try to use operator[] to set a bit, we'll get a compilation error.

This patch removes the 2 variants of Bitmap::operator[] and Bitmap::iterator::operator=.