Update dev/release/README for master releases, remove supporting scripts

[alamb]

Which issue does this PR close?

Closes #1120

Rationale for this change

If we go with the changes proposed in #1120, there is no need for the release branch maintenance overhead

What changes are included in this PR?

1. Update docs
2. Remove backporting scripts

Are there any user-facing changes?

not directly

[codecov-commenter]

Codecov Report

Merging #1143 (00587ea) into master (719096b) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1143      +/-   ##
==========================================
- Coverage   82.55%   82.54%   -0.01%     
==========================================
  Files         169      169              
  Lines       50456    50456              
==========================================
- Hits        41655    41650       -5     
- Misses       8801     8806       +5     

Impacted Files	Coverage Δ
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (-0.46%)	⬇️
arrow/src/datatypes/field.rs	53.79% <0.00%> (-0.31%)	⬇️
arrow/src/array/transform/mod.rs	85.43% <0.00%> (-0.27%)	⬇️

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[liukun4515]

It reduces the overflow for releasing.

[liukun4515]

LGTM