Revert "Make gcc_solink_wrapper.py work with Python 3."

This reverts commit 8e8ed621.

Reason for revert: 
This seems to make link time of shared library extremely slow.

Repro
$ cat out/Release/args.gn
is_component_build = true
is_debug = false
enable_nacl = false
use_goma = true
# goma_dir = "~/goma_client/client/out/Release"
$ rm out/Release/libcontent.so
$ ninja -C out/Release libcontent.so # took more than 50 seconds vs 1 seconds without this CL

Original change's description:
> Make gcc_solink_wrapper.py work with Python 3.
> 
> Make sure |line| is a string and not a bytes object.
> 
> Bug: 941669
> Change-Id: Ief610b426326f849b49353c5cbce68deb0cd4b3b
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2260872
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Auto-Submit: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#781381}

TBR=thakis@chromium.org,raphael.kubo.da.costa@intel.com

Change-Id: I67e0c5889212531c7eed913ce4f79448365281e0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 941669
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2262363Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781740}

build/toolchain/gcc_solink_wrapper.py

@@ -25,7 +25,6 @@ def CollectSONAME(args):
   readelf = subprocess.Popen(wrapper_utils.CommandToRun(
       [args.readelf, '-d', args.sofile]), stdout=subprocess.PIPE, bufsize=-1)
   for line in readelf.stdout:
-    line = line.decode('utf-8')
     if 'SONAME' in line:
       toc += line
   return readelf.wait(), toc
@@ -40,7 +39,6 @@ def CollectDynSym(args):
       stdout=subprocess.PIPE,
       bufsize=-1)
   for line in nm.stdout:
-    line = line.decode('utf-8')
     toc += ' '.join(line.split(' ', 2)[:2]) + '\n'
   return nm.wait(), toc