chore: Remove obsolete preview build support

[Gudahtt]

Description

We had previously configured CI to support installing @metamask- scoped packages from the GitHub npm registry, as we used this registry for "preview builds" from the core repository in the past. This is no longer used; we publish preview builds to npm now instead. The obsolete CI changes have been removed.

Related issues

• Preview build instructions (permalink)
• Preview build related extension PRs: Migrate to new controller packages #16547, Fix preview builds on CI #19970, Fix non-preview draft PR builds #20096, Fix prep-deps CI step #20312

Manual testing steps

This is not anticipated to have any functional impact. If it does break something, it would be the install step, so we can test this by installing locally (and seeing that the install step succeeded on CI).

Screenshots/Recordings

N/A

Pre-merge author checklist

• I've followed MetaMask Contributor Docs and MetaMask Extension Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable
• I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.

[metamaskbot]

Builds ready [9886674]

• builds: chrome, firefox
• builds (beta): chrome
• builds (flask): chrome, firefox
• builds (MMI): chrome, firefox
• builds (test): chrome, firefox
• builds (test-flask): chrome, firefox
• build viz: Build System
• mv3: Background Module Init Stats
• mv3: UI Init Stats
• mv3: Module Load Stats
• mv3: Bundle Size Stats
• mv2: E2e Actions Stats
• code coverage: Report
• storybook: Storybook
• typescript migration: Dashboard
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6
  • common: 0, 1, 10, 11, 2, 3, 4, 5, 6, 7, 8, 9
  • content-script: 0
  • offscreen: 0
  • ui: 0, 1, 10, 11, 12, 2, 3, 4, 5, 6, 7, 8, 9

Page Load Metrics (2054 ± 54 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	340	2320	1965	386	186
		domContentLoaded	1814	2205	2005	94	45
		load	1826	2331	2054	112	54
		domInteractive	18	88	58	22	11
		backgroundConnect	11	127	48	34	17
		firstReactRender	53	129	97	20	9
		getState	6	62	21	18	9
		initialActions	0	1	0	0	0
		loadScripts	1304	1587	1463	66	32
		setupStore	12	70	29	19	9
		uiStartup	2024	2706	2271	134	64

Bundle size diffs

• background: 0 Bytes (0.00%)
• ui: 0 Bytes (0.00%)
• common: 0 Bytes (0.00%)