build, test: Remove unused TIMEOUT environment variable

Setting the `TIMEOUT` environment variable has been a noop in both cases
since its introduction.

It seems to have been inadvertently copy-pasted from existing code. For
example, in commit d80e3cb, it was
needlessly copied from a valid case a few line above for the
`qa/pull-tester/run-bitcoind-for-test.sh` script.

Makefile.am

@@ -186,7 +186,7 @@ baseline_filtered.info: baseline.info
 	$(LCOV) -a $@ $(LCOV_OPTS) -o $@
 
 fuzz.info: baseline_filtered.info
-	@TIMEOUT=15 test/fuzz/test_runner.py $(DIR_FUZZ_SEED_CORPUS) -l DEBUG
+	@test/fuzz/test_runner.py $(DIR_FUZZ_SEED_CORPUS) -l DEBUG
 	$(LCOV) -c $(LCOV_OPTS) -d $(abs_builddir)/src --t fuzz-tests -o $@
 	$(LCOV) -z $(LCOV_OPTS) -d $(abs_builddir)/src
 
@@ -204,7 +204,7 @@ test_bitcoin_filtered.info: test_bitcoin.info
 	$(LCOV) -a $@ $(LCOV_OPTS) -o $@
 
 functional_test.info: test_bitcoin_filtered.info
-	@TIMEOUT=15 test/functional/test_runner.py $(EXTENDED_FUNCTIONAL_TESTS)
+	@test/functional/test_runner.py $(EXTENDED_FUNCTIONAL_TESTS)
 	$(LCOV) -c $(LCOV_OPTS) -d $(abs_builddir)/src --t functional-tests -o $@
 	$(LCOV) -z $(LCOV_OPTS) -d $(abs_builddir)/src