doc: remove redundant optionals from build-unix.md

Calling dependencies optional in a list of already optional dependencies
is redundant.

doc/build-unix.md

@@ -42,12 +42,12 @@ Optional dependencies:
  ------------|------------------|----------------------
  miniupnpc   | UPnP Support     | Firewall-jumping support
  libnatpmp   | NAT-PMP Support  | Firewall-jumping support
- libdb4.8    | Berkeley DB      | Optional, wallet storage (only needed when legacy wallet enabled)
+ libdb4.8    | Berkeley DB      | Wallet storage (only needed when legacy wallet enabled)
  qt          | GUI              | GUI toolkit (only needed when GUI enabled)
- libqrencode | QR codes in GUI  | Optional for generating QR codes (only needed when GUI enabled)
- libzmq3     | ZMQ notification | Optional, allows generating ZMQ notifications (requires ZMQ version >= 4.0.0)
- sqlite3     | SQLite DB        | Optional, wallet storage (only needed when descriptor wallet enabled)
- systemtap   | Tracing (USDT)   | Optional, statically defined tracepoints
+ libqrencode | QR codes in GUI  | QR code generation (only needed when GUI enabled)
+ libzmq3     | ZMQ notification | ZMQ notifications (requires ZMQ version >= 4.0.0)
+ sqlite3     | SQLite DB        | Wallet storage (only needed when descriptor wallet enabled)
+ systemtap   | Tracing (USDT)   | Statically defined tracepoints
 
 For the versions used, see [dependencies.md](dependencies.md)