Makefile: simplify `clean' rule

There is a special rule in make(1) to denote that we don't care about
the result of a command.  Additionally, we can also instruct rm(1) not
to complain when it didn't find a file/directory to remove.

As a final touch, there is no point of having 2 shell executions to
remove 2 or more files.  We can compress these 2 recipes into a single
one.

Reference:
   https://www.gnu.org/software/make/manual/html_node/Errors.html

Makefile

@@ -13,8 +13,7 @@ endif
 all: bin/pomo
 
 clean: 
-	rm -v bin/* 2> /dev/null || true
-	rm -v docs/* 2> /dev/null || true
+	-rm -fv bin/* docs/*
 
 bindata.go:
 	go-bindata -pkg main -o $@ tomato-icon.png