Bug 1241398 - Don't purge install manifests from the recursive make backend. r=gps

The code doing this was added before we had install manifests, back when
they were purge manifests, and before we tracked all files created by
the backend. Nowadays, if an install manifest is removed, it will be
removed in BuildBackend.consume.

In fact, purging the install manifests in the backend itself breaks the
deleted files count displayed after reticunating splines.

python/mozbuild/mozbuild/backend/recursivemake.py

@@ -18,7 +18,6 @@ from itertools import chain
 
 from reftest import ReftestManifest
 
-from mozpack.copier import FilePurger
 from mozpack.manifests import (
     InstallManifest,
 )
@@ -1342,18 +1341,10 @@ INSTALL_TARGETS += %(prefix)s
         man_dir = mozpath.join(self.environment.topobjdir, '_build_manifests',
             dest)
 
-        # We have a purger for the manifests themselves to ensure legacy
-        # manifests are deleted.
-        purger = FilePurger()
-
         for k, manifest in manifests.items():
-            purger.add(k)
-
             with self._write_file(mozpath.join(man_dir, k)) as fh:
                 manifest.write(fileobj=fh)
 
-        purger.purge(man_dir)
-
     def _write_master_test_manifest(self, path, manifests):
         with self._write_file(path) as master:
             master.write(

python/mozbuild/mozbuild/test/backend/test_recursivemake.py

@@ -566,6 +566,9 @@ class TestRecursiveMakeBackend(BackendTester):
         os.makedirs(purge_dir)
         m = InstallManifest()
         m.write(path=manifest_path)
+        with open(mozpath.join(
+                env.topobjdir, 'backend.RecursiveMakeBackend'), 'w') as f:
+            f.write('%s\n' % manifest_path)
 
         self.assertTrue(os.path.exists(manifest_path))
         self._consume('stub0', RecursiveMakeBackend, env)