Simplify GParted_Core::get_fs()

get_fs() used to work by (1) returning the supported capabilities of the
requested file system found in the FILESYSTEMS vector; (2) if not found
return the supported capabilities for file system FS_UNKNOWN; and (3)
if that wasn't found either, create a not supported capabilities set for
FS_UNKNOWN and return that.

This is more complicated that required.  Also the not supported
capabilities set, as created by struct FS() constructor, is the same as
that created in file_supported_filesystems() local variable fs_notsupp.

Simplify get_fs() just using a single not found code path returning a
not supported capabilities set.