Remove "TBA" runtime docs (#913)

Summary:
Pull Request resolved: #913

Didn't have a chance to write these before MVP. Remove and unlink them for now.

Reviewed By: mergennachin, larryliu0820

Differential Revision: D50290060

fbshipit-source-id: ed6707e535a128f36eef7aa71c01884137ae5eee

Author: dbort

docs/source/index.rst

@@ -143,8 +143,6 @@ Topics in this section will help you get started with ExecuTorch.
 
    runtime-overview
    runtime-backend-delegate-implementation-and-linking
-   runtime-custom-memory-allocator
-   runtime-error-handling
    runtime-platform-abstraction-layer
 
 .. toctree::

docs/source/runtime-custom-memory-allocator.md

@@ -159,8 +159,6 @@ For more details about the ExecuTorch runtime, please see:
 * [Runtime API Tutorial](running-a-model-cpp-tutorial.md)
 * [Runtime Build and Cross Compilation](runtime-build-and-cross-compilation.md)
 * [Runtime Platform Abstraction Layer](runtime-platform-abstraction-layer.md)
-* [Custom Memory Allocation](runtime-custom-memory-allocator.md)
-* [Runtime Error Handling](runtime-error-handling.md)
 * [Runtime Profiling](sdk-profiling.md)
 * [Backends and Delegates](compiler-delegate-and-partitioner.md)
 * [Backend Delegate Implementation](runtime-backend-delegate-implementation-and-linking.md)