There are two "allocator shims" on windows.
1) The deprecated one in allocator_shim_win.cc
2) The current one ("unified shim") in
   allocator_shim_override_ucrt_symbols_win.h
They work in the same exact way, that is, redefining ucrt
symbols like malloc/realloc and routing them to
winheap_stubs_win.cc.

The only reason of the dual shim is purely historical:
wfh@ was developing the VS2015 interposition-based shim while
primiano@ was developing the unified shim on other platforms.
Back then we consciously agreed to proceed in parallel as both
changes were risky.
Later, in crrev.com/2164653002 (54.0.2831.0) siggi@ made the work
to port the unified shim to Windows and enabled the flag that made
it default. Back then we decided to keep the old code around in
case of reverts.

The unified shim has been now been enabled by default for 9 months,
it is time to cleanup 1. This CL, hence, has no functional changes
(other than fixing #ifdefs in tests) and is just removing old
code that has not been used in the last 9 months.

BUG=550886
TBR=thakis

Review-Url: https://codereview.chromium.org/2907563002
Cr-Commit-Position: refs/heads/master@{#475012}