Don't build HTML pages for provided man pages. Instead, require users
to view them via `9 man`. For other software we also don't ship HTML
files for man pages.

Furthermore, there is a bug in plan9port where it would crash due to
a sprintf(3) buffer overflow detected by fortify-headers in the
following troff code:

	https://github.com/9fans/plan9port/blob/74374cc8a40c450423785015c2e30d5a2ed2920c/src/cmd/troff/n1.c#L247-L249

This code uses a fixed-size buffer and copies various data to this
buffer without a bounds check. This includes the path to the build
directory (DWBhomedir). On the builders this path itself is almost
100 bytes in size and hence causes a buffer overflow which then
causes fortify-headers to terminate troff execution with a SIGILL.

This causes all HTML pages, generated on the builders, to be broken.
Locally, this can't be reprouced since DWBhomedir will point to $PLAN9.
Also, this doesn't cause a build failure since the HTML pages are
generated by piping `troff -manhtml` into `troff2html` and this code is
executed without pipefail.

diff -upr a/INSTALL b/INSTALL
--- a/INSTALL	2021-10-19 18:08:45.000000000 +0200
+++ b/INSTALL	2022-04-16 12:56:44.710784569 +0200
@@ -175,12 +175,6 @@ if $doinstall; then
 		echo "* Renaming hard-coded /usr/local/plan9 paths..."
 		cd $PLAN9
 		sh lib/moveplan9.sh
-		echo "* Building web manual..."
-		(
-			cd $PLAN9/dist
-			echo cd `pwd`';' mk man
-			mk man
-		)
 	fi
 
 	if [ -x LOCAL.INSTALL ]; then