Don't use check_if_exists in InstallRequirement.uninstall (pypa#7422)

Previously InstallRequirement.uninstall was using
InstallRequirement.check_if_exists, which is a very overloaded
function with several callers that operate at different phases in
pip processing, not to mention that it mutates InstallRequirement itself.

Now we don't use that function for InstallRequirement.uninstall.

There should be no behavior change here.

src/pip/_internal/req/req_install.py

@@ -679,9 +679,8 @@ def update_editable(self, obtain=True):
                 % (self.link, vc_type))
 
     # Top-level Actions
-    def uninstall(self, auto_confirm=False, verbose=False,
-                  use_user_site=False):
-        # type: (bool, bool, bool) -> Optional[UninstallPathSet]
+    def uninstall(self, auto_confirm=False, verbose=False):
+        # type: (bool, bool) -> Optional[UninstallPathSet]
         """
         Uninstall the distribution currently satisfying this requirement.
 
@@ -694,10 +693,12 @@ def uninstall(self, auto_confirm=False, verbose=False,
         linked to global site-packages.
 
         """
-        if not self.check_if_exists(use_user_site):
+        assert self.req
+        try:
+            dist = pkg_resources.get_distribution(self.req.name)
+        except pkg_resources.DistributionNotFound:
             logger.warning("Skipping %s as it is not installed.", self.name)
             return None
-        dist = self.satisfied_by or self.conflicts_with
 
         uninstalled_pathset = UninstallPathSet.from_dist(dist)
         uninstalled_pathset.remove(auto_confirm, verbose)