fix(index): remove sourceMap warning

[yyx990803]

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why we should accept this pull request. If it fixes a bug or resolves a feature
request, be sure to link to that issue.

Currently as long as there is a source map passed from an upstream loader (e.g. vue-loader), postcss-loader warns about sourceMap being discarded even when the sourceMap option is explicitly set to false. It should only warn when the option is "not set".

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[coveralls]

Coverage increased (+0.9%) to 90.196% when pulling 2bb855c on yyx990803:patch-1 into 2484275 on postcss:master.

[coveralls]

Coverage increased (+0.07%) to 89.375% when pulling 367f180 on yyx990803:patch-1 into 2484275 on postcss:master.

[michael-ciniawsky]

Let's remove this warning altogether, it was added in good faith but folks use this loader in all various ways where an intermediated source map may be required for whatever reason... Please remove the notice from the docs aswell then https://github.com/postcss/postcss-loader#sourcemap (the blockquote starting with ⚠️ and following)

[yyx990803]

Done.

[michael-ciniawsky]

@yyx990803 Thx

[michael-ciniawsky]

Released in v2.1.5 🎉

[yoyo837]

Released in v2.0.5 v2.1.5 🎉