pxar::fuse: remove readdir implementation

pxar::fuse: remove readdir implementation

By not implementing readdir but only readdirplus, the FUSE_CAP_READDIRPLUS flag
is set while the FUSE_CAP_READDIRPLUS_AUTO flag is not set.
Thereby the kernel will issue only readdirplus calls.
Documentation at:
https://libfuse.github.io/doxygen/fuse-3_88_80_2include_2fuse__common_8h.html#a9b90333ad08d0e1c2ed0134d9305ee87

As the expensive part for accessing and reading the attributes is seeking and
decoding each directory entry, it is usefull to force readdirplus calls.
By this a struct `EntryParam` is returned for each entry, therebye avoiding a
subsequent lookup call.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>