Remove Cygwin-specific code from localcharset.c

That file had code for Cygwin < 1.7 and, apparently, Cygwin developers do not want to target Cygwin < 1.7 (1.7 released in 2009). Oblige them and remove that code. If anyone is using glib with Cygwin that is older than 2009, they would have to re-apply that patch again, possibly by copying the code from upstream localcharset.c, which still has that code 10 years later.