[CI] Remove skip_master feature

[leezu]

Description

Revert #685
We are currently only running tests with the master version of MXNet.

Propose to remove this feature, as it is dangerous to disable tests with the skip_master, as we may forget to enable them again. Instead, we can add a conditional skip based on the current date and an estimate when the issue is fixed in MXNet master.

cc @szha

[codecov]

Codecov Report

Merging #1017 into master will increase coverage by 0.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1017      +/-   ##
==========================================
+ Coverage   89.93%   90.52%   +0.58%     
==========================================
  Files          67       67              
  Lines        6340     6340              
==========================================
+ Hits         5702     5739      +37     
+ Misses        638      601      -37

Impacted Files	Coverage Δ
src/gluonnlp/model/bert.py	88.16% <0%> (-4.09%)	⬇️
src/gluonnlp/utils/files.py	45.09% <0%> (-3.93%)	⬇️
src/gluonnlp/model/sequence_sampler.py	91.63% <0%> (+17.07%)	⬆️

[mli]

Job PR-1017/2 is complete.
Docs are uploaded to http://gluon-nlp-staging.s3-accelerate.dualstack.amazonaws.com/PR-1017/2/index.html