riscv: cpufeature: Drop this_hwcap clear in T-Head vector workaround

riscv: cpufeature: Drop this_hwcap clear in T-Head vector workaround

The variable this_hwcap is initialized to 0 for each loop, it is not
necessary to do the bit clearance since this_hwcap is still 0 at this
point, clearing the source_isa is enough here.

Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://patch.msgid.link/20260430045350.22213-1-hui.wang@canonical.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>