INSTALL.Cygwin: Drop instructions to explicitly set MOCCMD
#41674 should make explicitly setting MOCCMD in cygwin build unnecessary. Remove the step from INSTALL.Cygwin build instructions.
Pushed to branches other than S3_1, which still lacks #41674.
#41674 should make explicitly setting MOCCMD in cygwin build unnecessary. Remove the step from INSTALL.Cygwin build instructions.