ptrs_list: simplify remove_at implementation

ptrs_list: simplify remove_at implementation

Consider a table with memory layout like this:

[A]->[B]->[C]->[D]

Now, consider the removal of B. Intuitively it would result in:

[A]------>[C]->[D]

Which is what will happen now. Previously, what happened was:

[A]->[C]------>[D]

meaning B wasn't actually removed; rather, C was removed and
B took on the value of C. This was not only needlessly complex
but also made it difficult to implement "safe" iteration
that lets one remove the value being iterated on, which is
going to be implemented in a subsequent patch.

Change-Id: Icca3066ecd7c6a33b3eaf9655780c1ffc97ba722
Signed-off-by: Michal Bloch <m.bloch@samsung.com>