Hemlock - SoD

**Title:** LUA warning spam from deprecated XML `<Backdrop>` on HemlockFrame

**Summary**
Opening a merchant repeatedly logs the warning:
`Hemlock/Hemlock.xml:55 Unrecognized XML attribute: top`
It fires once per merchant open (~15x in a session) and shows up in BugSack/BugGrabber.

**Cause**
`HemlockFrame` defines its background with an XML `<Backdrop>` block (Hemlock.xml lines 53–63), which uses the pre-8.0 backdrop system. Modern WoW clients (including Classic Era 1.15.9) removed native XML backdrops in favor of `BackdropTemplateMixin` in Lua, so the block no longer renders and the parser warns on its now-unrecognized attributes (e.g. `top` on `<AbsInset>`).

**Steps to reproduce**
1. Enable Lua error/warning display (`/console scriptErrors 1`) or run BugSack.
2. Log in on a rogue with Hemlock loaded.
3. Open any merchant.
4. Warning is logged; repeats on each subsequent merchant open.

**Expected**
No warning; the frame draws without generating parser noise.

**Actual**
`Hemlock.xml:55 Unrecognized XML attribute: top` logged repeatedly.

**Environment**
- Hemlock for SoD 1.1.6-sod
- WoW Classic Era / Season of Discovery, client 1.15.9 (interface 11509)

**Fix**
Removed the non-functional `<Backdrop>` block from `HemlockFrame` in Hemlock.xml (it wasn't rendering on modern clients anyway). No visual change. If a visible background is desired, reimplement it in Lua via `BackdropTemplateMixin`.