fix(compat): remove internal from import locations

[cartant]

Description:

This PR fixes the problem reported in #4070 by removing internal from the import locations within compat/Observable.ts. Note that no other import locations within compat include internal.

For more information:

• this comment shows the commit that introduced the problem; and
• this comment explains how the problem is effected in Angular-CLI projects.

Related issue (if exists): #4070

[coveralls]

Pull Request Test Coverage Report for Build 7958

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.813%

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Totals
Change from base Build 7954:	0.0%
Covered Lines:	5256
Relevant Lines:	5429

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💛 - Coveralls