fix(core): don't override alwaysShowVerticalScroll flag

[ghiscoding]

• the alwaysShowVerticalScroll flag should be dealt directly in the core and it should never show vertical scroll on left frozen container, even when the alwaysShowVerticalScroll is set to True (see core lib PR #537)
• requires core lib PR #537 to be merged and released

[codecov]

Codecov Report

Merging #600 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #600   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          149       149           
  Lines         9946      9945    -1     
  Branches      3516      3515    -1     
=========================================
- Hits          9946      9945    -1     

Impacted Files	Coverage Δ
.../angular-slickgrid/extensions/gridMenuExtension.ts	100.00% <100.00%> (ø)
...ngular-slickgrid/extensions/headerMenuExtension.ts	100.00% <100.00%> (ø)

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