Remove obsolete symbol from ecpg_config.h.in

Remove obsolete symbol from ecpg_config.h.in

HAVE_LONG_LONG_INT was not added to ecpg_config.h.in by the meson
build system, but rather than add it there, we decided to remove it
from the makefile build system, to make both consistent that way.

There is no documentation or examples that suggest that the presence
of this symbol was publicly advertised, and of course the feature is
required by C99 (but we don't necessarily require C99 for ecpg user
code).  ecpg core code and ecpg tests use the symbol
HAVE_LONG_LONG_INT_64 instead, which is still there.

Discussion: https://www.postgresql.org/message-id/flat/bf35d032-02fc-4173-9f4f-840999cc3ef3%40eisentraut.org