Remove override flag from ks env rm

[GuessWhoSamFoo]

Closes #822

This change ensures ks rm is consistent with ks env set with overriding.

• ks env rm no longer has an --override flag
• When overriding an env, it is not possible to change the non-overriden env.

Signed-off-by: GuessWhoSamFoo sfoohei@gmail.com

[bryanl]

@GuessWhoSamFoo how do you remove an override?

[coveralls]

Pull Request Test Coverage Report for Build 1276

• 7 of 9 (77.78%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 70.974%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/actions/env_rm.go	5	7	71.43%

Totals
Change from base Build 1275:	-0.02%
Covered Lines:	12050
Relevant Lines:	16978

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💛 - Coveralls

[GuessWhoSamFoo]

@bryanl ks env rm will default to the override if it exists.

[GuessWhoSamFoo]

Closed per discussion on UX around this