New warning: Unused imports should be removed

[Cheshiriks]

What's done:

• Added new warning
• Added new test

Which rule and warnings did you add?

• UNUSED_IMPORT

This pull request closes #710

Actions checklist

• Implemented Rule, added Warnings
• Added tests on checks
• Added tests on fixers
• Updated diktat-analysis.yml
• Updated available-rules.md

[codecov]

Codecov Report

Merging #717 (33cb3e7) into master (5dd1394) will decrease coverage by 0.04%.
The diff coverage is 73.52%.

@@             Coverage Diff              @@
##             master     #717      +/-   ##
============================================
- Coverage     80.00%   79.96%   -0.05%     
- Complexity     1947     1958      +11     
============================================
  Files            93       93              
  Lines          5007     5040      +33     
  Branches       1606     1618      +12     
============================================
+ Hits           4006     4030      +24     
  Misses          245      245              
- Partials        756      765       +9     

Flag	Coverage Δ	Complexity Δ
unittests	79.96% <73.52%> (-0.05%)	0.00 <11.00> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
...lin/org/cqfn/diktat/plugin/maven/DiktatBaseMojo.kt	78.94% <ø> (ø)	0.00 <0.00> (ø)
.../kotlin/org/cqfn/diktat/plugin/maven/DiktatMojo.kt	80.00% <ø> (ø)	0.00 <0.00> (ø)
...at-rules/src/main/kotlin/generated/WarningNames.kt	0.00% <ø> (ø)	0.00 <0.00> (ø)
...leset/rules/chapter2/comments/HeaderCommentRule.kt	82.79% <ø> (ø)	42.00 <0.00> (ø)
...tat/ruleset/rules/chapter3/files/WhiteSpaceRule.kt	78.34% <ø> (ø)	141.00 <0.00> (ø)
.../cqfn/diktat/ruleset/utils/FunctionAstNodeUtils.kt	66.66% <ø> (ø)	0.00 <0.00> (ø)
.../kotlin/org/cqfn/diktat/ruleset/utils/KdocUtils.kt	15.78% <ø> (ø)	0.00 <0.00> (ø)
.../ruleset/rules/chapter3/files/FileStructureRule.kt	78.66% <67.85%> (-2.64%)	74.00 <11.00> (+11.00)	⬇️
...tlin/org/cqfn/diktat/ruleset/constants/Warnings.kt	97.77% <100.00%> (+0.01%)	12.00 <0.00> (ø)
...tlin/org/cqfn/diktat/ruleset/utils/AstConstants.kt	100.00% <100.00%> (ø)	0.00 <0.00> (ø)

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[petertrr]

lgtm, fix one comment and you can merge (don't forget to squash and merge)