Remove all instances of precertificate signing CAs from the TLS BRs

[ryancdickson]

Assuming SC-092 (discussion period) goes to vote and passes, sometime after March 16, 2026, we should plan to remove all TLS BR references to precertificate signing CAs.

This issue was motivated by #615 (comment), described during the SC-092 pre-balloting process.

[github-actions]

This issue was created based on:

• TLS BR Version 2.1.7
• EVG Version 2.0.1

[dzacharo]

@ryancdickson how long should we allow this language in the TBRs considering there are such Subordinate CAs still valid?