[scenes/opengl] Remove outdated hack to reset vertex buffers

Summary:
Scene opengl has a callback for when we have a GL error. One of the
handlers for an error calls scheduleVboReInit the history shows it was a
forerunner to the GLX_NV_robustness_video_memory_purge but resetting
only one tiny part based on debug output.

When we get here we schedule a reset of the vertex buffer, via a timer.
When the timer is caled we have no idea what GL context was last
current, if it's not the currect context then the main scene
GLVertexBuffer will be deleted but not correctly re-initialised.

We have two very common crashes with a corrupted
GLVertexBuffer::streamingBuffer() which would match up perfectly.

Given that we now have a proper mechanism to reset the entire scene, we
don't need this timer based hack and resolve that problem.

BUG: 399499
BUG: 372305

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D26556