build: Remove old and unused PGO_BUILD buildflag.

It's a remnant of the old Windows-only MSVC-based PGO attempt.

Bug: 309849
Change-Id: I353b848b79399bbdc1d3f1d183dc04a610a63e00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390940
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Cr-Commit-Position: refs/heads/master@{#803879}

chrome/common/BUILD.gn

@@ -54,7 +54,6 @@ buildflag_header("buildflags") {
     "ENABLE_TAB_SEARCH=$enable_tab_search",
     "ENABLE_WAYLAND_SERVER=$enable_wayland_server",
     "ENABLE_WEBUI_TAB_STRIP=$enable_webui_tab_strip",
-    "PGO_BUILD=$pgo_build",
     "OPTIMIZE_WEBUI=$optimize_webui",
   ]
 }

chrome/common/features.gni

@@ -62,9 +62,6 @@ declare_args() {
   # Indicates if Wayland display server support is enabled.
   enable_wayland_server = is_chromeos
 
-  # Indicates if the build is using PGO.
-  pgo_build = chrome_pgo_phase > 0
-
   # optimize_webui was moved to ui/base/ui_features.gni
 }