electrumx: fix removing conn from pool

[joshuasing]

Summary
Set p.pool to return value of slices.DeleteFunc when removing a connection.
As of go1.23, go vet reports this issue: https://go-review.googlesource.com/c/tools/+/554317

The item is still removed from the slice, however the length of the slice is not updated. See implementation of slices.DeleteFunc:

	clear(s[i:]) // zero/nil out the obsolete elements, for GC
	return s[:i]

This has likely been causing errors with the pool, such as connections not being returned and instead being closed, as well as the size being returned wrong.

Changes

• Set p.pool to return value of slices.DeleteFunc when removing a connection.