common: remove dead code

Each symbol below has no references anywhere in the tree (src/, contrib/,
tests/), and none on the in-flight FCMP++ (fcmp++-stage-new) or CARROT
(carrot_impl) integration branches; the full build still links cleanly.
Per-symbol history is given so the "why is this unused" is auditable (hashes
are ancestors of HEAD).

command_line:
- parse_command_line template: boost wrapper used by the connectivity tool and
  wallet from the 2014 import; the last caller was removed in b8a08f199 (2017,
  "wallet: fix --log-file not working"). Not boost's own
  boost::program_options::parse_command_line, a different function still in use.

download:
- download_finished / download_error: dead on arrival - added 63f0e074e (2017,
  "download: async API") but never wired to any caller.

perf_timer:
- set_performance_timer_log_level: dead on arrival - added 74dfdb0b3 (2016),
  never called. It was the only writer of performance_timer_log_level, which is
  therefore never reassigned and stays at its default (el::Level::Info).

Note: the unused PERF_TIMER_L / PERF_TIMER_UNIT_L macros are deliberately left
in place - the FCMP++ wallet-scanning work (fcmp++-stage-new, wallet2.cpp) uses
PERF_TIMER_L, so removing them would break that branch on rebase.

src/common/download.cpp

@@ -274,20 +274,6 @@ namespace tools
     return control;
   }
 
-  bool download_finished(const download_async_handle &control)
-  {
-    CHECK_AND_ASSERT_MES(control != 0, false, "NULL async download handle");
-    boost::lock_guard<boost::mutex> lock(control->mutex);
-    return control->stopped;
-  }
-
-  bool download_error(const download_async_handle &control)
-  {
-    CHECK_AND_ASSERT_MES(control != 0, false, "NULL async download handle");
-    boost::lock_guard<boost::mutex> lock(control->mutex);
-    return !control->success;
-  }
-
   bool download_wait(const download_async_handle &control)
   {
     CHECK_AND_ASSERT_MES(control != 0, false, "NULL async download handle");