arch: x86: Remove not used fp struct

The legacy struct s_coopFloatReg was never being used, though it was
an empty struct (not wasting space), some symbols were being generate
for it.

Nevertheless, neither C99 nor C11 allow empty structs, so this
was also a violation to the C standards.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>