Proposal: Remove speed tests if registry is specified

[pd4d10]

What problem does this feature solve?

vue-cli would do a speed test and choose the faster registry:

vue-cli/packages/@vue/cli/lib/util/shouldUseTaobao.js

Lines 65 to 68 in 1a0b591

	faster = await Promise.race([
	ping(defaultRegistry),
	ping(registries.taobao)
	])

While registry is specified, this test seems useless because in most cases users would prefer the registry specified explicitly.

What does the proposed API look like?

Remove speed tests while registry is set in npmrc or yarnrc.

[pd4d10]

Seems related to #3997