Simplify build:

- remove needless src/modules.c, that generates by nginx build system
- rename the module's source file to src/ngx_otel_module.cpp

src/modules.c

@@ -1,14 +0,0 @@
-#include <ngx_config.h>
-#include <ngx_core.h>
-
-extern ngx_module_t gHttpModule;
-
-ngx_module_t* ngx_modules[] = {
-    &gHttpModule,
-    NULL
-};
-
-char* ngx_module_names[] = {
-    "ngx_http_otel_module",
-    NULL
-};