Remove deprecated {{.IMAGES}} and {{.DIGEST_}} env vars

[nkubala]

These variables were marked for deprecation well over a year ago - it's time they were removed.

[codecov]

Codecov Report

Merging #5605 (0e46886) into master (8b4cfda) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5605      +/-   ##
==========================================
- Coverage   71.09%   71.04%   -0.05%     
==========================================
  Files         402      402              
  Lines       15183    15167      -16     
==========================================
- Hits        10794    10776      -18     
- Misses       3596     3597       +1     
- Partials      793      794       +1     

Impacted Files	Coverage Δ
pkg/skaffold/build/custom/script.go	51.16% <ø> (-1.11%)	⬇️
pkg/skaffold/tag/tag.go	100.00% <ø> (ø)
pkg/skaffold/tag/env_template.go	100.00% <100.00%> (ø)
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️
pkg/skaffold/docker/parse.go	85.40% <0.00%> (+1.08%)	⬆️

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[gsquared94]

lgtm!