DOC: Remove Series.sortlevel from api.rst (#23395)
DOC: Remove Series.sortlevel from api.rst (#23395)

Was dropped in gh-15099 but accidentally added back in gh-18202.

Follow-up to gh-23375.
cbertinato/pandas,GuessWhoSamFoo/pandas,toobaz/pandas,TomAugspurger/pandas,MJuddBooth/pandas,toobaz/pandas,jorisvandenbossche/pandas,rs2/pandas,toobaz/pandas,GuessWhoSamFoo/pandas,pandas-dev/pandas,GuessWhoSamFoo/pandas,datapythonista/pandas,toobaz/pandas,pandas-dev/pandas,dsm054/pandas,jreback/pandas,pandas-dev/pandas,MJuddBooth/pandas,TomAugspurger/pandas,gfyoung/pandas,jreback/pandas,gfyoung/pandas,jreback/pandas,MJuddBooth/pandas,jreback/pandas,dsm054/pandas,cbertinato/pandas,GuessWhoSamFoo/pandas,dsm054/pandas,harisbal/pandas,MJuddBooth/pandas,dsm054/pandas,datapythonista/pandas,harisbal/pandas,jreback/pandas,harisbal/pandas,datapythonista/pandas,rs2/pandas,GuessWhoSamFoo/pandas,jorisvandenbossche/pandas,dsm054/pandas,MJuddBooth/pandas,TomAugspurger/pandas,gfyoung/pandas,gfyoung/pandas,jorisvandenbossche/pandas,TomAugspurger/pandas,cbertinato/pandas,rs2/pandas,harisbal/pandas,pandas-dev/pandas,jorisvandenbossche/pandas,rs2/pandas,cbertinato/pandas,harisbal/pandas,toobaz/pandas,gfyoung/pandas,cbertinato/pandas,datapythonista/pandas