doc: Use man_MANS in Makefile.am

automake does know where to install manual pages, so use man_MANS
instead of hand crafting custom rules.

docs/man/Makefile.am

@@ -1,14 +1,8 @@
-EXTRA_DIST = sesman.ini.5 xrdp.ini.5 xrdp.8 xrdp-sesman.8 xrdp-sesrun.8
-
-xrdpman5dir=$(mandir)/man5
-
-xrdpman5_DATA = \
+man_MANS = \
   sesman.ini.5 \
-  xrdp.ini.5
-
-xrdpman8dir=$(mandir)/man8
-
-xrdpman8_DATA = \
+  xrdp.ini.5 \
   xrdp.8 \
   xrdp-sesman.8 \
   xrdp-sesrun.8
+
+EXTRA_DIST = $(man_MANS)