docs/variants/msi: remove Full Build release statements

[pietrushnic]

Summary

• remove Full Build problem statements from the MSI PRO Z690-A v1.1.7 release
  entry
• remove the equivalent copied statements from the MSI PRO Z790-P v0.9.5
  release entry
• keep both release entries focused on release-specific changes, artifacts,
  verification, and SBOM information

Content placement

No prose was relocated. The accurate general subscription and support benefits
already remain in the introduction of each release page, while the supported
variants overview is the canonical location for the platform support model.
The removed text claimed that these platforms were available exclusively as
Full Builds and that a standalone Dasharo Pro Package was unavailable, which
conflicts with that overview. It therefore had no legitimate documentation
target.

After this change, the newest release entries proceed directly from their
binary lists to signature-verification instructions and contain no Full Build
problem statements.

Verification

• pre-commit run --files docs/variants/msi_z690/releases.md docs/variants/msi_z790/releases.md
• mkdocs build
• verified the rendered Z690 and Z790 release pages contain no Full Build
  problem-statement wording

Follow-up noted during verification

The Z790 page title and introduction scope the page to DDR5, while the v0.9.5
entry includes DDR4-specific change and fix bullets and publishes both DDR4
and DDR5 artifacts. This pre-existing release-data inconsistency is intentionally
left unchanged here pending confirmation from the release owner.