Revert "Native: enable K2 for benchmarks in performance/"

This workaround is not needed anymore -- bootstrap KGP is now K2.

This reverts commit 4672727bd6.

kotlin-native/performance/gradle.properties

@@ -18,9 +18,5 @@ artifactoryRepo = kotlin-native-benchmarks
 externalReports = coroutinesReport.txt
 externalBenchmarksReport = externalReport.json
 
-# This can be reverted once bootstrap is updated to 2.0.0:
-kotlin.experimental.tryK2=true
-# See commit message for more details.
-
 # Avoid building platform libraries by the MPP plugin.
 kotlin.native.distribution.type=prebuilt