Remove component selectors

[tkh44]

What:
Close #329

Why:

The idea is neat, but the amount of code and effort required (maintenance) do not meet its utility.

The babel plugin is no longer required for basic usage.*

* the plugin enables the following features

• styled.div syntax (styled('div') will work without the plugin)
• minification
• /*#__PURE__*/ flag injection
• static extraction
• source maps

Checklist:

• Documentation
• Tests
• Code complete

[codecov]

Codecov Report

Merging #334 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
- Coverage    97.3%   97.27%   -0.03%     
==========================================
  Files          17       17              
  Lines         593      551      -42     
  Branches      142      130      -12     
==========================================
- Hits          577      536      -41     
+ Misses         12       11       -1     
  Partials        4        4

Impacted Files	Coverage Δ
packages/react-emotion/src/index.js	100% <100%> (ø)	⬆️
packages/babel-plugin-emotion/src/index.js	94% <100%> (-0.78%)	⬇️

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