build: Remove build_pigweed_python_source_tree from default
build: Remove build_pigweed_python_source_tree from default

The build_pigweed_python_source_tree step is fairly slow since it
involves installing Python packages. The step runs for every Python
change. Remove it from the default build target since it doesn't provide
helpful feedback during development.

The build_pigweed_python_source_tree target is still very simple to run:

  ninja -C out pw_env_setup:build_pigweed_python_source_tree

Change-Id: Ie7fcce11bed73fed2e6309654161c37f929337e8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/60448
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed