MAINT: Simplify _read_fmt_chunk bytes read / res
MAINT: Simplify _read_fmt_chunk bytes read / res

res was being assigned to but not used

Based on the code at:
https://github.com/scipy/scipy/pull/6852/commits/a70a6007f0e5ceb3371d86565bc4492a26c589d6#diff-c5c9583b55bf7580040d99e32c4f1771L64

matthew-brett/scipy,WarrenWeckesser/scipy,anntzer/scipy,Stefan-Endres/scipy,endolith/scipy,pizzathief/scipy,grlee77/scipy,rgommers/scipy,nmayorov/scipy,scipy/scipy,WarrenWeckesser/scipy,scipy/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,e-q/scipy,e-q/scipy,Eric89GXL/scipy,ilayn/scipy,WarrenWeckesser/scipy,tylerjereddy/scipy,zerothi/scipy,nmayorov/scipy,matthew-brett/scipy,pizzathief/scipy,grlee77/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,mdhaber/scipy,matthew-brett/scipy,perimosocordiae/scipy,vigna/scipy,ilayn/scipy,Eric89GXL/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,ilayn/scipy,pizzathief/scipy,vigna/scipy,endolith/scipy,endolith/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,tylerjereddy/scipy,zerothi/scipy,Stefan-Endres/scipy,rgommers/scipy,andyfaff/scipy,andyfaff/scipy,mdhaber/scipy,nmayorov/scipy,endolith/scipy,grlee77/scipy,perimosocordiae/scipy,pizzathief/scipy,vigna/scipy,scipy/scipy,endolith/scipy,zerothi/scipy,Eric89GXL/scipy,scipy/scipy,andyfaff/scipy,tylerjereddy/scipy,e-q/scipy,anntzer/scipy,mdhaber/scipy,rgommers/scipy,mdhaber/scipy,grlee77/scipy,ilayn/scipy,e-q/scipy,pizzathief/scipy,Eric89GXL/scipy,andyfaff/scipy,anntzer/scipy,tylerjereddy/scipy,andyfaff/scipy,zerothi/scipy,mdhaber/scipy,anntzer/scipy,Stefan-Endres/scipy,anntzer/scipy,e-q/scipy,ilayn/scipy,zerothi/scipy,grlee77/scipy,nmayorov/scipy,scipy/scipy,tylerjereddy/scipy,scipy/scipy,zerothi/scipy,perimosocordiae/scipy,ilayn/scipy,vigna/scipy,nmayorov/scipy,mdhaber/scipy,perimosocordiae/scipy,andyfaff/scipy,Eric89GXL/scipy,matthew-brett/scipy,vigna/scipy,matthew-brett/scipy,rgommers/scipy,WarrenWeckesser/scipy,endolith/scipy,rgommers/scipy,anntzer/scipy