common/strtol: do not check for existence of <charconv>

common/strtol: do not check for existence of <charconv>

<charconv> is available since GCC-8, see https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017

> Elementary string conversions	P0067R5	11.1 (integral types supported since 8.1)	__has_include(<charconv>), __cpp_lib_to_chars >= 201611

since we always have the access to GCC-8.1 and up, there is no need to
detect the existence of <charconv> anymore.

also, because GCC-11 introduced the support of float types support,
update the comment to reflect the change.

Signed-off-by: Kefu Chai <kchai@redhat.com>