Revert "pw_env_setup: Support requirements in JSON"

This reverts commit 6bfc7bae3bfb748dad478a6e92e753d43d828dea.

Reason for revert: not needed after pwrev/29488

Original change's description:
> pw_env_setup: Support requirements in JSON
>
> Support requirements.txt files in env setup JSON. Leave out of docs
> since we discourage using it.
>
> Change-Id: I5de894f2e9dcd8468bdc27f48431c20cf41dc562
> Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/37240
> Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
> Reviewed-by: Armando Montanez <amontanez@google.com>
> Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>

TBR=mohrr@google.com,hepler@google.com,amontanez@google.com,pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com,auto-submit@pigweed.google.com.iam.gserviceaccount.com

Change-Id: Ic6b97d39cea841b653b123f9ab57c450022c53c1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/37286
Reviewed-by: Rob Mohr <mohrr@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>
Commit-Queue: Rob Mohr <mohrr@google.com>
diff --git a/pw_env_setup/docs.rst b/pw_env_setup/docs.rst
index 698a4c2..7832a9f 100644
--- a/pw_env_setup/docs.rst
+++ b/pw_env_setup/docs.rst
@@ -180,13 +180,6 @@
   only installing Pigweed Python packages, use the location of the Pigweed
   submodule.
 
-``virtualenv.requirements``
-  A ``requirements.txt`` file as written by ``pip-compile``, relative to
-  ``PW_PROJECT_ROOT``. Listing multiple files is supported but unlikely to
-  work because the same package may be pinned to two different versions. Best
-  practice is to include dependencies in ``setup.py`` files and not just in
-  ``requirements.txt``.
-
 An example of a config file is below.
 
 .. code-block:: json
@@ -201,9 +194,6 @@
       "gn_root": ".",
       "gn_targets": [
         ":python.install",
-      ],
-      "requirements": [
-        "tools/requirements.txt"
       ]
     }
   }
diff --git a/pw_env_setup/py/pw_env_setup/env_setup.py b/pw_env_setup/py/pw_env_setup/env_setup.py
index 62a3ae5..b3f1fdd 100755
--- a/pw_env_setup/py/pw_env_setup/env_setup.py
+++ b/pw_env_setup/py/pw_env_setup/env_setup.py
@@ -254,8 +254,6 @@
 
         virtualenv = config.pop('virtualenv', {})
 
-        self._virtualenv_requirements = virtualenv.pop('requirements', [])
-
         if virtualenv.get('gn_root'):
             root = os.path.join(self._project_root, virtualenv.pop('gn_root'))
         else: