Refactor away ModuleTestBase::assertTableCount()

Problem/Motivation

Discovered via #3204764: PHPUnit assertions do not return a value, ModuleTestBase::assertTableCount() is only called once in core:

core/modules/system/tests/src/Functional/Module/DependencyTest.php
48:    $this->assertTableCount('language', FALSE);

It is never called with the second argument being TRUE.

It is also unused in contrib: http://grep.xnddx.ru/search?text=assertTableCount&filename=

Steps to reproduce

Proposed resolution

Refactor the single caller to use tableExists() instead and remove the method.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet