Remove BITS_IN_BYTES define
    
    While working on the previous patch, I noticed that BITS_IN_BYTES can be
    replaced by HOST_CHAR_BIT, which is used more widely in gdb.
    
    ChangeLog
    2017-08-14  Tom Tromey  <tom@tromey.com>
    
    	* valprint.c (print_octal_chars): Use HOST_CHAR_BIT.
    	(print_binary_chars): Likewise.
    	(BITS_IN_BYTES): Remove.