ash: exec: Never rehash regular built-ins

Upstream commit:

    Date: Sat, 19 May 2018 02:39:51 +0800
    exec: Never rehash regular built-ins

    As regular (including special) built-ins can never be overridden,
    we should never remove them from the hash table.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>