test: Remove deleted --only-covered infection option (#27)

It only mutates covered files by default now.

Author: theofidry

Makefile

@@ -12,7 +12,7 @@ COVERAGE_HTML = $(COVERAGE_DIR)/html
 TARGET_MSI = 100
 
 INFECTION_BIN = vendor/bin/infection
-INFECTION = php -d zend.enable_gc=0 $(INFECTION_BIN) --skip-initial-tests --coverage=$(COVERAGE_DIR) --only-covered --show-mutations --min-msi=100 --min-covered-msi=100 --ansi
+INFECTION = php -d zend.enable_gc=0 $(INFECTION_BIN) --skip-initial-tests --coverage=$(COVERAGE_DIR) --show-mutations --min-msi=100 --min-covered-msi=100 --ansi
 
 PHPUNIT_BIN = vendor/bin/phpunit
 PHPUNIT = php -d zend.enable_gc=0 $(PHPUNIT_BIN)