Variables: Unconditionally deprecate old vars engine extensions

[medikoo]

When investigating code I've noticed a weird not logical condition which is expected to lead to deprecation when external plugin attempts to provide a new source for old variables engine.

This patch fixes that

[codecov]

Codecov Report

Merging #9827 (4889d08) into master (dc74f41) will not change coverage.
The diff coverage is 0.00%.

❗ Current head 4889d08 differs from pull request most recent head 2979638. Consider uploading reports for the commit 2979638 to get more accurate results

@@           Coverage Diff           @@
##           master    #9827   +/-   ##
=======================================
  Coverage   86.14%   86.14%           
=======================================
  Files         329      329           
  Lines       12804    12804           
=======================================
  Hits        11030    11030           
  Misses       1774     1774           

Impacted Files	Coverage Δ
...riables/sources/resolve-external-plugin-sources.js	88.23% <0.00%> (ø)

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[pgrzesik]

good catch 👍