Remove sync_pipfile

See also: tmux-python/tmuxp@2e01c32

Author: tony

Makefile

@@ -32,9 +32,3 @@ flake8:
 
 watch_flake8:
 	if command -v entr > /dev/null; then ${PY_FILES} | entr -c $(MAKE) flake8; else $(MAKE) flake8 entr_warn; fi
-
-sync_pipfile:
-	pipenv install --skip-lock --dev -r requirements/doc.txt && \
-	pipenv install --skip-lock --dev -r requirements/dev.txt && \
-	pipenv install --skip-lock --dev -r requirements/test.txt && \
-	pipenv install --skip-lock --dev -e .