fix: avoid to declare and export NbTable component

[adwd]

Please read and mark the following check list before creating a pull request:

• I read and followed the CONTRIBUTING.md guide.
• I read and followed the New Feature Checklist guide.

Short description of what this resolves:

NbTable is only used as base class of NbTreeGridComponent but is is declared and exported. Because of this, cdk-table cannot be used.
This PR removes NbTable from declare/export array to fix it.

<!-- in app using Nebular -->
<table cdk-table [dataSource]="dataSource">
  ...
</table>

More than one component matched on this element.
Make sure that only one component's selector can match a given element.
Conflicting components: NbTable,CdkTable

[codecov]

Codecov Report

Merging #2126 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2126   +/-   ##
=======================================
  Coverage   81.53%   81.53%           
=======================================
  Files         244      244           
  Lines        7377     7377           
  Branches      773      773           
=======================================
  Hits         6015     6015           
  Misses       1135     1135           
  Partials      227      227           

Impacted Files	Coverage Δ
...amework/theme/components/cdk/table/table.module.ts	100.00% <ø> (ø)

[yggg]

@adwd Thanks for the PR. Unfortunately, we can't keep the component decorator without adding it to the module. Instead, I changed NbTable component selector to a different one. It'll fix the issue as well.