graphics/qgis-ltr, graphics/qgis: Fix build after textproc/txt2tags update

The FreeBSD package builder complains about encoding problems after [1]. This happens, because newer versions of textproc/txt2tags do not need the option '--encoding=utf-8' any more[2].

The patch is a workaround to remove such encoding entries from the QGIS port, until QGIS project changes upstream for newer txt2tags versions.

While I'm here, replace sed commands by a patch file [3]

[1] http://svnweb.freebsd.org/changeset/ports/525495
[2] https://github.com/txt2tags/txt2tags/blob/v3/CHANGELOG.md#35-2020-02-01
[3] https://reviews.freebsd.org/D23188#511597

PR: 243998, 243996
Submitted by: Rainer Hurling (maintainer)
Reviewed by: lbartoletti
Approved by: tcbernor (mentor)
Differential Revision: https://reviews.freebsd.org/D23602