delete unused ECL_LIB_GIT_VERSION

Since 'ecl' is nomore a git submodule, code lines related to
'ECL_LIB_GIT_VERSION' could be deleted

src/lib/version/version.h

@@ -179,11 +179,6 @@ __EXPORT const char *px4_firmware_git_branch(void);
  */
 __EXPORT uint64_t px4_firmware_version_binary(void);
 
-/**
- * ECL lib version as human readable string (git tag)
- */
-__EXPORT const char *px4_ecl_lib_version_string(void);
-
 /**
  * MAVLink lib version in binary form (first part of the git tag)
  */