graphics/librsvg2: Remove PORTREVISON (typo) variable

ports r470310 updated graphics/librsvg2 (PORTVERSION) to 2.40.20, but also
included a typo'd PORTREVISION variable, with a value of 1, in the change.

Since the PORTVERSION was updated, incrementing the PORTREVISION was not
necessary as the updated package would have been built without it.

Since the typo'd variable is not included in PKGNAME version calculation,
it is a noop.

Remove the variable accordingly.

PR: 233214
Reported by: John Hein <z7dr6ut7gs snkmail com>
Approved by: portmgr (just fix it)
MFH: 2018Q4