DOC: Remove Series.sortlevel from api.rst

Was dropped in pandas-devgh-15099 but accidentally added back in pandas-devgh-18202.

Follow-up to pandas-devgh-23375.

Author: gfyoung

doc/source/api.rst

@@ -515,7 +515,6 @@ Reshaping, sorting
    Series.repeat
    Series.squeeze
    Series.view
-   Series.sortlevel
 
 
 Combining / joining / merging