Floss: remove speed optimization in MMC test build

The "speed" optimization is only necessary when a fuzzer is involved.
Removing this as it is a suspected cause of increased build time.

Bug: 317993722
Test: m Bluetooth; time FEATURES=test emerge-$BOARD floss
Flag: EXEMPT floss only changes
Change-Id: I8900036204456cf95797c8397c4602b635027411