Remove additional flex alt text

[MrOrz]

const FLEX_MESSAGE_ALT_TEXT in handlers/util asks the user to proceed action on their mobile phone, if their device does not support flex messages.

At this point of time, all devices, including desktop, supports flex messages. The only chance the user can see the alt text is probably the short quote in the chatroom list, which only displays the first few letters.

When implementing new submission flow, I removed FLEX_MESSAGE_ALT_TEXT and updates each of its occurrences. This is submitted as a separate PR (this PR) in order to make the main PR more focused.

[coveralls]

Pull Request Test Coverage Report for Build 2216308322

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.008%) to 87.343%

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Totals
Change from base Build 2197145743:	-0.008%
Covered Lines:	982
Relevant Lines:	1109

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💛 - Coveralls

[nonumpa]

LGTM!