soc: cv32a6: Remove erroneous CPU_HAS_FPU configs

In the default configuration, cv32a6 does not have an FPU and does not
implement RISC-V's F and D extensions.
Hence, the FPU flags should not be added.
In the future, a second SoC for cv32a6 systems with FPU can be added.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>