metal_dcache_l1_discard() does not require fence.i

[TacoGrandeTX]

Our metal_dcache_l1_discard() contains a fence.i instruction that is not needed and should be removed:

https://github.com/sifive/freedom-metal/blob/v201908-branch/src/cache.c#L161

Invalidating a line in the data cache or the entire data cache has no impact on instruction side accesses and invalidating the I Cache can have a heavy performance impact.