kconfig: remove 'e1' and 'e2' macros from expression deduplication

I do not think the macros 'e1' and 'e2' are readable.

The statement:

    e1 = expr_alloc_symbol(...);

affects the caller's variable, but this is not sufficiently clear from the code.

Remove the macros. No functional change intended.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>