Maint: Remove duplicate path extension code

Previously, the Windows exec provider had incorrect logic for
resolving an executable using a hard-coded list of extensions,
e.g. '.cmd' wasn't in the list.

Recently, Puppet::Util.which was modified to perform path resolution
taking into account the PATHEXT environment variable on Windows. See
0258096c474d1b2cbf7e403fc990a853ac277231. As a result, the duplicate
logic in the Windows exec provider has been removed.