erofs: kill use_vmap module parameter

As Christoph said [1],
"vm_map_ram is supposed to generally behave better.  So if
it doesn't please report that that to the arch maintainer
and linux-mm so that they can look into the issue.  Having
user make choices of deep down kernel internals is just
a horrible interface.

Please talk to maintainers of other bits of the kernel
if you see issues and / or need enhancements. "

Let's redo the previous conclusion and kill the vmap
approach.

[1] https://lore.kernel.org/r/20190830165533.GA10909@infradead.org/
Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190904020912.63925-21-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/filesystems/erofs.txt

@@ -67,10 +67,6 @@ cache_strategy=%s      Select a strategy for cached decompression from now on:
                                    It still does in-place I/O decompression
                                    for the rest compressed physical clusters.
 
-Module parameters
-=================
-use_vmap=[0|1]         Use vmap() instead of vm_map_ram() (default 0).
-
 On-disk details
 ===============