Support removing colorbar ticks in GR and PGFPlotsX

[ivan-boikov]

Description

Related #4659
I also took the liberty of rearranging assembly of colorbar parameters in PGFPlotsX to reduce code redundancy (it also helped for the patch). Hope that's alright.

Testing code

testplot = (; kwargs...) -> plot([0,1]; label=:none, zcolor=[0,1], kwargs...)
plot(
    testplot(),
    testplot(; colorbar_ticks=:none)
)

Before fix

After fix

Attribution

• I am listed in .zenodo.json (see How to get acknowledged #3503)

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (fbdaa89) 90.42% compared to head (d6363b8) 90.41%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4704      +/-   ##
==========================================
- Coverage   90.42%   90.41%   -0.01%     
==========================================
  Files          40       40              
  Lines        8719     8723       +4     
==========================================
+ Hits         7884     7887       +3     
- Misses        835      836       +1     

Impacted Files	Coverage Δ
src/backends/gr.jl	90.68% <100.00%> (+<0.01%)	⬆️
src/backends/pgfplotsx.jl	88.29% <100.00%> (-0.12%)	⬇️

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