[feat] Drop support of pipeline stage method override

[teojgo]

Fixes #1352

[codecov-io]

Codecov Report

Merging #1636 (0a21974) into master (edfd903) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1636   +/-   ##
=======================================
  Coverage   87.54%   87.54%           
=======================================
  Files          44       44           
  Lines        7288     7288           
=======================================
  Hits         6380     6380           
  Misses        908      908           

Impacted Files	Coverage Δ
reframe/core/meta.py	96.96% <100.00%> (ø)
reframe/core/pipeline.py	92.28% <100.00%> (ø)
reframe/frontend/cli.py	74.94% <0.00%> (ø)

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[vkarak]

lgtm