Remove error diagnostics uniqueness check and .json generation.

[pnkfelix]

Remove error diagnostics uniqueness check and .json generation.

This is yet another revamping of PR #25592, more in line with its original strategy.

As @alexcrichton said on that PR:

the syntax diagnostic validation as it's been plaguing the bots for two separate reasons and needs a quick fix for now:

• It's possible to read the stale metadata of previous builds, causing spurious failures that can only be solved by blowing away tmp.
• The code does not currently handle concurrent compilations where JSON files in the metadata directory may be half-written and hence contain invalid JSON when read.

This is meant to be a temporary measure to get the builds to be reliable again; see also Issue #25705.

[rust-highfive]

r? @alexcrichton

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[nikomatsakis]

@bors r+

[bors]

📌 Commit deaa117 has been approved by nikomatsakis

[nikomatsakis]

@bors p=22

this is needed by bots

[brson]

@bors force

[brson]

@bors clean

[bors]

⌛ Testing commit deaa117 with merge 1725fe4...

[pnkfelix]

@bors force

[pnkfelix]

force merging; AFAICT this passes the tier1 platform tests on buildbot, and something else has gone wrong that is keeping it from being merged. (e.g. perhaps homu is confused.)