syscall: drop fallback to utimes in UtimesNano on Linux

syscall: drop fallback to utimes in UtimesNano on Linux

The minimum required Linux kernel version for Go 1.18 will be changed to
2.6.32, see #45964. The current minimum required version is 2.6.23 and
utimensat was added in 2.6.22, so the fallback isn't even necessary for
the current minimum supported version. Remove the fallback to utimes.

For #45964

Change-Id: I5536f6ea7a34944dd9165f1533c10692171fb0c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/346790
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>