Remove dead code from margin and element_text setup

margin.setup() assigned themeable_name to self but the attribute was
never read anywhere in the repo, and element_text.setup() reassigned
m to the return value of m.setup(...) which is None. Both are no-ops
that only work because margin.setup mutates in place.

Author: has2k1

plotnine/themes/elements/element_text.py

@@ -189,7 +189,7 @@ def setup(self, theme: theme, themeable_name: str):
         Setup the theme_element before drawing
         """
         if m := self.properties.get("margin"):
-            m = m.setup(theme, themeable_name)
+            m.setup(theme, themeable_name)
 
     def _translate_hjust(
         self, just: float

plotnine/themes/elements/margin.py

@@ -70,7 +70,6 @@ def setup(self, theme: theme, themeable_name: str):
         convert them to different units as is required. Here we get
         all the parameters that we shall need to do the conversions.
         """
-        self.themeable_name = themeable_name
         self.fontsize = theme.getp((themeable_name, "size"), 11)
         self.figure_size = theme.getp("figure_size")
         self._is_setup = True