Merge bitcoin/bitcoin#26089: build: remove unused cppflags

4b656b9ed3 build: remove unused libevent cppflags (fanquake)
afce044bb6 build: remove unused natpmp / upnp cppflags (fanquake)

Pull request description:

  Their inclusion is likely just the result of copy-paste.

  The only place upnp & natpmp CPPFLAGS  should be used is [`libbitcoin_node` (mapport.cpp)](https://github.com/bitcoin/bitcoin/blob/13fd9ee5c2d747e9f74d3fd8e33a4f0c9eaa769c/src/Makefile.am#L352).

ACKs for top commit:
  theuni:
    ACK 4b656b9ed3
  hebasto:
    ACK 4b656b9ed3, I have reviewed the code and it looks OK, I agree it can be merged.

Tree-SHA512: 800daeb06ddcbade3a862ca939a8cf87dc36282ed4fe9dc256d17b78a8751e2e08799788dc449046e1d875f93372912269d3ffcb5702628a41648794df32e887

src/Makefile.test_fuzz.include

@@ -13,7 +13,7 @@ TEST_FUZZ_H = \
     test/fuzz/mempool_utils.h \
     test/fuzz/util.h
 
-libtest_fuzz_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BOOST_CPPFLAGS) $(MINIUPNPC_CPPFLAGS) $(NATPMP_CPPFLAGS) $(EVENT_CFLAGS) $(EVENT_PTHREADS_CFLAGS)
+libtest_fuzz_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BOOST_CPPFLAGS)
 libtest_fuzz_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 libtest_fuzz_a_SOURCES = \
   test/fuzz/fuzz.cpp \

src/Makefile.test_util.include

@@ -20,7 +20,7 @@ TEST_UTIL_H = \
     test/util/validation.h \
     test/util/wallet.h
 
-libtest_util_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BOOST_CPPFLAGS) $(MINIUPNPC_CPPFLAGS) $(NATPMP_CPPFLAGS) $(EVENT_CFLAGS) $(EVENT_PTHREADS_CFLAGS)
+libtest_util_a_CPPFLAGS = $(AM_CPPFLAGS) $(BITCOIN_INCLUDES) $(BOOST_CPPFLAGS)
 libtest_util_a_CXXFLAGS = $(AM_CXXFLAGS) $(PIE_FLAGS)
 libtest_util_a_SOURCES = \
   test/util/blockfilter.cpp \