Remove unused constants in block.module

Since we no longer have the per user block functionality in core these constants are no longer required.

/**
 * Users cannot control whether or not they see this block.
 */
const BLOCK_CUSTOM_FIXED = 0;

/**
 *  Shows this block by default, but lets individual users hide it.
 */
const BLOCK_CUSTOM_ENABLED = 1;

/**
 * Hides this block by default but lets individual users show it.
 */
const BLOCK_CUSTOM_DISABLED = 2;

I'll be adding them into the contrib module for the per user block feature: see #1871854: Decide whether to restore per-user block visibility or provide it in contrib