Remove unnecessary (char *) casts [mem]

Remove unnecessary (char *) casts [mem]

Remove (char *) casts around memory functions such as memcmp(),
memcpy(), or memset() where the cast is useless.  Since these
functions don't take char * arguments anyway, these casts are at best
complicated casts to (void *), about which see commit 7f798aca1d5.

Reviewed-by: Dagfinn Ilmari Mannsåker 
Discussion: https://www.postgresql.org/message-id/flat/fd1fcedb-3492-4fc8-9e3e-74b97f2db6c7%40eisentraut.org