chore: remove external header target

there is no need to update the header file, and we already patched it in
925745f0e3 so new version won't work
unless patched

compiler/Makefile

@@ -5,8 +5,6 @@ PARALLEL_EXECUTION_ENABLED=OFF
 CC_COMPILER=
 CXX_COMPILER=
 
-EXTERNAL_HEADERS=include/boost-single-header/outcome.hpp
-
 KEYSETCACHEDEV=/tmp/KeySetCache
 KEYSETCACHECI=../KeySetCache
 
@@ -63,9 +61,6 @@ $(BUILD_DIR)/configured.stamp:
 	-DPython3_EXECUTABLE=${Python3_EXECUTABLE}
 	touch $@
 
-include/boost-single-header/outcome.hpp:
-	wget https://github.com/ned14/outcome/raw/master/single-header/outcome.hpp -O $@
-
 build-initialized: $(EXTERNAL_HEADERS) $(BUILD_DIR)/configured.stamp
 
 doc: build-initialized