hugolib: Remove summary.go
hugolib: Remove summary.go

Became obsolete after refactoring.

Fixes #2888.
fedelibre/hugo,tubo028/hugo,mitchchn/hugo,anthonyfok/hugo,fedelibre/hugo,gohugoio/hugo,mitchchn/hugo,fedelibre/hugo,digitalcraftsman/hugo,mdhender/hugo,mdhender/hugo,anthonyfok/hugo,gohugoio/hugo,fedelibre/hugo,tubo028/hugo,digitalcraftsman/hugo,mdhender/hugo,gohugoio/hugo,mdhender/hugo,anthonyfok/hugo