kconfig: remove compat_getline()

kconfig: remove compat_getline()

Commit 1a7a8c6fd8ca ("kconfig: allow long lines in config file") added
a self-implemented getline() for better portability.

However, getline() is standardized [1] and already used in other programs
such as scripts/kallsyms.c.

Use getline() provided by libc.

[1]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getdelim.html

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