build: Drop dead non-pkg-config code for libevent check

configure.ac

@@ -1297,27 +1297,12 @@ CPPFLAGS="$TEMP_CPPFLAGS"
 
 fi
 
-if test x$use_pkgconfig = xyes; then
-  : dnl
-  m4_ifdef(
-    [PKG_CHECK_MODULES],
-    [
-      if test x$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench != xnonononono; then
-        PKG_CHECK_MODULES([EVENT], [libevent >= 2.0.21], [use_libevent=yes], [AC_MSG_ERROR(libevent version 2.0.21 or greater not found.)])
-        if test x$TARGET_OS != xwindows; then
-          PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads >= 2.0.21],, [AC_MSG_ERROR(libevent_pthreads version 2.0.21 or greater not found.)])
-        fi
-      fi
-    ]
-  )
-else
+dnl libevent check
 
-  if test x$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench != xnonononono; then
-    AC_CHECK_HEADER([event2/event.h], [use_libevent=yes], AC_MSG_ERROR(libevent headers missing),)
-    AC_CHECK_LIB([event],[main],EVENT_LIBS=-levent,AC_MSG_ERROR(libevent missing))
+if test x$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench != xnonononono; then
+  PKG_CHECK_MODULES([EVENT], [libevent >= 2.0.21], [use_libevent=yes], [AC_MSG_ERROR([libevent version 2.0.21 or greater not found.])])
   if test x$TARGET_OS != xwindows; then
-      AC_CHECK_LIB([event_pthreads],[main],EVENT_PTHREADS_LIBS=-levent_pthreads,AC_MSG_ERROR(libevent_pthreads missing))
-    fi
+    PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads >= 2.0.21],, [AC_MSG_ERROR([libevent_pthreads version 2.0.21 or greater not found.])])
   fi
 fi