Remove ASM_UNIMPLEMENTED from Builtins::Generate_FunctionCall

Remove ASM_UNIMPLEMENTED from Builtins::Generate_FunctionCall

It's actually called by several tests, for example with

var a = {}; a.call = Function.prototype.call; a.call();

In contrast to the _BREAK() variant ASM_UNIMPLEMENTED() doesn't trigger
any visible output, so it's not readily obvious that this code path
is actually taken

BUG=none
R=baptiste.afsa@arm.com, ulan@chromium.org
LOG=n

Committed: https://code.google.com/p/v8/source/detail?r=18963

[ulan, 2014-01-30 16:43:41]

lgtm

[baptiste.afsa1, 2014-01-30 17:04:23]

lgtm

[jochen (gone - plz use gerrit), 2014-01-30 19:01:39]

Committed patchset #1 manually as r18963 (presubmit successful).