make: drop the `-c` arg to `install` in the documentation makefile

This arg caused `gmake install` on OpenIndiana 2019.10 (illumos) fail with:

    install: The -c, -f, -n options each require a directory following!
    install: The -c, -f, -n options each require a directory following!
    install: The -c, -f, -n options each require a directory following!
    gmake[1]: *** [Makefile:41: install] Error 2
    gmake[1]: Leaving directory '/usr/local/share/mercurial/doc'

The workaround is to run `gmake install-bin`.

The man page for 10.14 says this is to copy the file and is only for
compatability, as it is the default.  The CentOS 7 man page says it is ignored.
The top level makefile doesn't use this argument at all, so I'm not sure why
it's here.

Differential Revision: https://phab.mercurial-scm.org/D8439

doc/Makefile

@@ -5,7 +5,7 b' GENDOC=gendoc.py ../mercurial/commands.p'
 	../mercurial/helptext/*.txt ../hgext/*.py ../hgext/*/__init__.py
 PREFIX=/usr/local
 MANDIR=$(PREFIX)/share/man
-INSTALL=install -c -m 644
+INSTALL=install -m 644
 PYTHON?=python
 RSTARGS=