finance/gnucash: Fix packaging when guile3 is being used

After commit 7edcfaeb079e3285b299ef619ab9d09d46c447a2 the guile
version being depended upon can be configured by users via
DEFAULT_VERSIONS.

This patch adapts the plist to this option. [1]

While here, also remove some unneeded dependencies. [2]

PR: 274001 [1]
Reported by: poudriere [2]