Remove entries of MathTextParser._backend_mapping deprecated in 3.4.

00ad9c4 replaced the entries for "bitmap", etc. to
MathtextBackendAgg/MathtextBackendPath (from the earlier
MathtextBackendBitmap, etc., which had been deprecated in 3.4), but the
intent was actually to remove these entries as well upon deprecation
expiration; e.g. in 3.5 `MathTextParser("bitmap").parse("foo")` raises a
DeprecationWarning.

Author: anntzer

lib/matplotlib/mathtext.py

@@ -202,13 +202,8 @@ class MathTextParser:
     _parser = None
 
     _backend_mapping = {
-        'bitmap': MathtextBackendPath,
         'agg':    MathtextBackendAgg,
-        'ps':     MathtextBackendPath,
-        'pdf':    MathtextBackendPath,
-        'svg':    MathtextBackendPath,
         'path':   MathtextBackendPath,
-        'cairo':  MathtextBackendPath,
         'macosx': MathtextBackendAgg,
     }
     _font_type_mapping = {