Download refactor (#2275)

* remote: drop not working http resume

Closes #2136

* remote: drop @transfer_context()

It is not needed for ssh anymore and GS/S3 are switched to client
instances in cached properties.

* remote: drop upload/download chunking

scripts/completion/dvc.bash

@@ -26,7 +26,7 @@ _dvc_fetch='--show-checksums -j --jobs -r --remote -a --all-branches -T --all-ta
 _dvc_get_url=''
 _dvc_get='-o --out --rev'
 _dvc_gc='-a --all-branches -T --all-tags -c --cloud -r --remote -f --force -p --projects -j --jobs'
-_dvc_import_url='--resume -f --file'
+_dvc_import_url='-f --file'
 _dvc_import='-o --out --rev'
 _dvc_init='--no-scm -f --force'
 _dvc_install=''

scripts/completion/dvc.zsh

@@ -133,7 +133,6 @@ _dvc_gc=(
 )
 
 _dvc_importurl=(
-  "--resume[Resume previously started download.]"
   {-f,--file}"[Specify name of the DVC-file it generates.]:File:_files"
   "1:URL:"
   "2:Output:"