remove context method for disabling parallel builds

There is no reason to disable them once they are enabled and it may not
be safe.

Author: dhellmann

src/fromager/context.py

@@ -80,9 +80,6 @@ def __init__(
     def enable_parallel_builds(self) -> None:
         self._parallel_builds = True
 
-    def disable_parallel_builds(self) -> None:
-        self._parallel_builds = False
-
     @property
     def wheels_build(self) -> pathlib.Path:
         # when parallel builds are enabled, return a path that is unique for the