Update isValid generator and regenerate SPIRVIsValidEnum.h

For many enums in `spirv.hpp`, an isValid function was generated in
`SPIRVIsValidEnum.h` even though that function was never used.  Remove
them to reduce build times and maintenance burden.

If a removed isValid function is needed again in the future, the enum
name needs to be added back to `gen_spirv.bash` followed by a rerun of
the script.

Regenerate SPIRVIsValidEnum.h using the updated `gen_spirv.bash`
script.

Author: svenvh