cdecl99: Drop locale-sensitive isblank usage.

cdecl99: Drop locale-sensitive isblank usage.

The intention of this function is to avoid recording no-op command lines
in the history.  This is exactly the set of commands containing just
regular tabs and spaces.

It is inappropriate to use the locale-sensitive isblank for this, as
this may be a little bit different.  In practice there is probably no
meaningful difference, but as isblank is a C99 feature losing this call
also helps when building against older C libraries that lack it.