network: Remove the cursed libslirp fork now that Windows uses external libslirp

.ci/AppImageBuilder.yml

@@ -57,7 +57,7 @@ AppDir:
     - libqt5gui5 # if QT:BOOL=ON
     - libqt5widgets5 # if QT:BOOL=ON
     - libsixel1 # if CLI:BOOL=ON
-    - libslirp0 # if SLIRP_EXTERNAL:BOOL=ON
+    - libslirp0
     - libsndio7.0 # if OPENAL:BOOL=ON
     - libvdeplug-dev # -dev also pulls in libvdeplug2. -dev is required to get the proper .so symlink to the library
     - libx11-6 # if QT:BOOL=ON

.ci/build.sh

@@ -670,9 +670,6 @@ EOF
 	else
 		echo [-] Not installing dependencies again
 	fi
-
-	# Link against the system libslirp instead of compiling ours.
-	cmake_flags_extra="$cmake_flags_extra -D SLIRP_EXTERNAL=ON"
 fi
 
 # Point CMake to the toolchain file.