cmake: Remove ENABLE_ARM_SHANI from bitcoin-build-config.h

`ENABLE_ARM_SHANI` is already conditionally defined for the
`bitcoin_crypto` target, so defining it in `bitcoin-build-config.h` is
redundant.

cmake/introspection.cmake

@@ -217,5 +217,4 @@ if(NOT MSVC)
     " HAVE_ARM_SHANI
     CXXFLAGS ${ARM_SHANI_CXXFLAGS}
   )
-  set(ENABLE_ARM_SHANI ${HAVE_ARM_SHANI})
 endif()