fix: remove default value for devtool

[anshumanv]

What kind of change does this PR introduce?
fix

Did you add tests for your changes?
NA

If relevant, did you update the documentation?
NA

Summary
Remove the default value of some flags since they are invalid and not used in most cases.

Does this PR introduce a breaking change?
No

Other information

[codecov]

Codecov Report

Merging #2053 (428194d) into master (1e0b0b4) will decrease coverage by 0.50%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2053      +/-   ##
==========================================
- Coverage   68.81%   68.31%   -0.51%     
==========================================
  Files          82       77       -5     
  Lines        2437     2408      -29     
  Branches      494      496       +2     
==========================================
- Hits         1677     1645      -32     
- Misses        760      763       +3     

Impacted Files	Coverage Δ
packages/webpack-cli/lib/utils/cli-flags.js	100.00% <ø> (ø)
packages/webpack-cli/lib/webpack-cli.js	90.19% <0.00%> (-3.93%)	⬇️
packages/webpack-cli/lib/utils/arg-parser.js	93.47% <0.00%> (ø)
packages/webpack-cli/lib/groups/resolveStats.js
packages/webpack-cli/lib/groups/resolveMode.js
packages/webpack-cli/lib/groups/basicResolver.js
packages/webpack-cli/lib/groups/resolveOutput.js
packages/webpack-cli/lib/groups/resolveAdvanced.js

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[snitin315]

the color needs some more fixes, I'm removing it in
#2042 (comment)

[snitin315]

Let's remove it for devtool only here.

[webpack-bot]

@anshumanv Thanks for your update.

I labeled the Pull Request so reviewers will review it again.

@snitin315 Please review the new changes.