allocator: Remove tcmalloc override when build_for_tool==memcheck

Practical reason of this CL:
This is causing a breakage in the Mac Valgrind builder
after crrev.com/1616793003:
https://build.chromium.org/p/chromium.memory.fyi/builders/Chromium%20Mac%20Builder%20%28valgrind%29/builds/91232/steps/runhooks/logs/stdio

Longer explanation:
crrev.com/1616793003 made allocator target toolset: host, target
(which itself should be OK)

A very old condition was forcing use_allocator=tcmalloc on all OS
when build_for_tool=memcheck.
This causes problems now on the Mac valgrind bot when building the
chromium_builder_dbg_valgrind_mac target.
In the last change that touched this line, crrev.com/258433005, the
original condition was linux_use_tcmalloc=1. This was mistakenly
translated in a blanket allocator==tcmalloc.

Rationale for removing this line:
allocator=tcmalloc is the default on linux these days. I don't see a
reason why re-enforcing this condition, as use_allocator=tcmalloc by
default on the right (Linux/Android) platforms.

BUG=564618,580108
TEST=gyp_chromium -Dbuild_for_tool=memcheck && ninja chromium_builder_dbg_valgrind_mac

Review URL: https://codereview.chromium.org/1613183002

Cr-Commit-Position: refs/heads/master@{#370724}

build/common.gypi

@@ -2300,7 +2300,6 @@
         'win_release_InlineFunctionExpansion': '0',
         'win_release_OmitFramePointers': '0',
 
-        'use_allocator': 'tcmalloc',
         'release_valgrind_build': 1,
         'werror': '',
         'component': 'static_library',