Makefile: remove .SILENT, make the targets loud

The Makefile was set globally to silent, which caused the failing
commands to be *not* displayed. For example

flo@neo-pc xeps-xsf $ make html
make: *** [Makefile:112: build/xep-0001.html] Error 1

Removing this provides usefull information about which command failed:

flo@neo-pc xeps-xsf $ make html
xmllint --nonet --noout --noent --loaddtd --valid "xep-0001.xml"
! xmllint --nonet --noout --noent --loaddtd --xpath "//img/@src[not(starts-with(., 'data:'))]" xep-0001.xml 2>/dev/null && true
make: *** [Makefile:110: build/xep-0001.html] Error 1

Makefile

@@ -1,5 +1,3 @@
-.SILENT:
-
 OUTDIR?=build
 REFSDIR?=$(OUTDIR)/refs
 EXAMPLESDIR?=$(OUTDIR)/examples