Remove non-existing variable reference in MSVC's Solution.pm
The version string is grabbed from PACKAGE_VERSION in pg_config.h in the
MSVC build since
8f4fb4c6, but an error message referenced a variable
that existed before that. This had no consequences except if one messes
up enough with the version number of the build.
Author: Anton Voloshin
Discussion: https://postgr.es/m/
af79ee1b-9962-b299-98e1-
f90a289e19e6@postgrespro.ru
Backpatch-through: 13