Disable PGO for Release builds (#17765)

Same justification as #17749.

We will revert this when either OneBranch Custom Pools become
fit-for-purpose or they upgrade to VS 17.11. Or the heat death of the
universe.

(cherry picked from commit 628e99f)
Service-Card-Id: PVTI_lADOAF3p4s4AmhmszgSGCaA
Service-Version: 1.21

Author: DHowett

build/pipelines/ob-release.yml

@@ -27,7 +27,7 @@ parameters:
   - name: pgoBuildMode
     displayName: "PGO Build Mode"
     type: string
-    default: Optimize
+    default: None # BODGY - OneBranch is on VS 17.10, which is known to be the worst
     values:
       - Optimize
       - Instrument