Partially revert "add target html and gallinahtml"

This partially reverts 2bf11f4f.  There is no need to add new Makefile targets as the Makefile already forwards all targets to Makefile.coq.
2 jobs for master in 1 minute and 59 seconds
Status Job ID Name Coverage
  Test
passed #2714
fp-timing
stdpp-coq8.5

00:01:59

passed #2715
fp-timing
stdpp-coq8.6

00:01:34