Linux 5.2 compat: Remove config/kernel-set-fs-pwd.m4

This failed on 5.2-rc1 with "error: unknown" message, for set_fs_pwd()
not being visible in both const and non-const tests.

This is caused by torvalds/linux@83da1bed86. It's configurable,
but we would want to be able to compile with default kbuild setting.

set_fs_pwd() has never been exported with exception of some distro
kernels, and set_fs_pwd() wasn't used in ZoL to begin with. The test
result was used for a spl function vn_set_fs_pwd().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #8777