selftests/nolibc: drop NOLIBC_SYSROOT=0 logic

selftests/nolibc: drop NOLIBC_SYSROOT=0 logic

This logic was added in commit 850fad7de827 ("selftests/nolibc: allow
test -include /path/to/nolibc.h") to allow the testing of -include
/path/to/nolibc.h. As it requires as special variable to activate, this
code is nearly never used. Furthermore it complicates the logic a bit.

Since commit a6a054c8ad32 ("tools/nolibc: add target to check header
usability") and commit 443c6467fcd6 ("selftests/nolibc: always run
nolibc header check") the usability of -include /path/to/nolibc.h is
always checked anyways, making NOLIBC_SYSROOT=0 pointless.

Drop the special logic.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260104-nolibc-nolibc_sysroot-v1-1-98025ad99add@weissschuh.net