Remove outdated note about tracing.exportPerformanceMetrics

This parameter no longer exists. This note was accidentally restored in
change I65790d0b1. Since the performance metrics are only recorded for
configured operations now, there is no longer a need for a global config
switch to turn them off.

Release-Notes: skip
Change-Id: Ifcab1cd7600c716f3139aa563f47acef41a5b476
Signed-off-by: Edwin Kempin <ekempin@google.com>

Documentation/metrics.txt

diff --git a/Documentation/metrics.txt b/Documentation/metrics.txt
index a9b82d6..a510e25 100644
--- a/Documentation/metrics.txt
+++ b/Documentation/metrics.txt
@@ -93,10 +93,6 @@
 ** `plugin`:
    The name of the plugin that performed the operation.
 
-Performance metrics can be enabled via the
-link:config.gerrit.html#tracing.exportPerformanceMetrics[`tracing.exportPerformanceMetrics`]
-setting.
-
 === Pushes
 
 * `receivecommits/changes`: histogram of number of changes processed in a single