Don't delete new dalvik-cache for otapreopt

otapreopt would incorrectly clear out the entire
/data/ota/$BOOT_SLOT/dalvik-cache/$ISA directory each time it compiled
an apk. This meant that pre-installed apks (from /system, /product,
etc) would have their compilation artifacts deleted during OTAs,
causing them to fallback to running from the APK or extracted dexs.

Bug: 181182967
Test: Manual OTA of blueline

Change-Id: I414424a8963e18e01b892a6e7ba2fb1beeb1a38d