Don't set b_coverage by default

The coverage tracker tinkers with the binary leaving traces behind even in
release builds.

.drone.yml

@@ -7,7 +7,7 @@ steps:
   image: arminfriedl/xwim-build
   commands:
     - meson wrap install gtest
-    - meson build
+    - meson build -Db_coverage=true
     - ninja -C build
     - ninja -C build test && ninja -C build coverage
     - echo "******** TEST LOGS ***********"

meson.build

@@ -2,7 +2,6 @@ project('xwim', 'cpp',
         version: '0.3',
         default_options: ['cpp_std=c++17',
                           'warning_level=3',
-                          'b_coverage=true',
                           'b_ndebug=if-release'])
 
 subdir('src')