Revert "define builder_cache base path"

This reverts commit e718958ca0.

Reason for revert: Suspected to have caused an outage http://o/370111
It seems to have shifted the current dir to a new place, created another checkout and some bots with small disk space ran out of disk.

Original change's description:
> define builder_cache base path
> 
> chromium_tests recipe module currently checks if builder_cache base path
> is defined. If it is not, it let's the caller to decide where to put
> the cache. All the callers choose start_dir.
> https://cs.chromium.org/search/?q=get_checkout_dir&type=cs
> 
> In LUCI we want to use [CACHE]/builder as builder cache. To do that we
> want to update chromium_tests recipe module to use [CACHE]/builder if
> builder_cache is not defined.
> 
> Define builder_cache in buildbot to point to start dir, so that in LUCI
> it is not defined and thus [CACHE]/builder is used.
> 
> R=​iannucci@chromium.org
> BUG=660481
> 
> Change-Id: I9018da5c43a5396add6a26ad2c12b1755fbe6307
> Reviewed-on: https://chromium-review.googlesource.com/457439
> Reviewed-by: Ryan Tseng <hinoka@chromium.org>
> Commit-Queue: Nodir Turakulov <nodir@chromium.org>
> 

TBR=iannucci@chromium.org,hinoka@chromium.org,estaab@chromium.org,nodir@chromium.org,chromium-reviews@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=660481

Change-Id: Ia239ebe12a5bea1987c2ee23e543460efe753c81
Reviewed-on: https://chromium-review.googlesource.com/457778
Reviewed-by: Sergey Berezin <sergeyberezin@chromium.org>
Commit-Queue: Sergey Berezin <sergeyberezin@chromium.org>

recipe_modules/infra_paths/path_config.py

@@ -16,7 +16,6 @@ def infra_buildbot(c):
   c.base_paths['root'] = c.START_DIR[:-4]
   c.base_paths['cache'] = c.base_paths['root'] + (
       'build', 'slave', 'cache')
-  c.base_paths['builder_cache'] = c.START_DIR
   c.base_paths['git_cache'] = c.base_paths['root'] + (
       'build', 'slave', 'cache_dir')
   c.base_paths['goma_cache'] = c.base_paths['root'] + (