com32: Delete execute.c and use the one from ldlinux

com32: Delete execute.c and use the one from ldlinux

ldlinux provides us with an execute.c and all the functions that we
need, there's no point in duplicating the code in com32/menu.

Also, the copy in ldlinux doesn't use __com32.cs_bounce. Using
cs_bounce no longer works with dynamic ELF modules because we don't
have such tight control over the address space. Instead, memory must
be allocated dynamically or reserved as part of the object file.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>