I created an environment with openjdk
When I activated the env, I had the same echo as you (except with my path)
In anaconda3\envs\tc35 look for etc\conda\activate.d
In mine, I found java_home.bat with the contents
set "JAVA_HOME_CONDA_BACKUP=%JAVA_HOME%"
set "JAVA_HOME=%CONDA_PREFIX%\Library"
Deleting java_home.bat stopped the echo when activating my env, but I'm not certain the package will work correctly after.
It's possible to comment out set "JAVA_HOME_CONDA_BACKUP=%JAVA_HOME%". See conda-forge/openjdk-feedstock to verify this doesn't lead to any adverse results.
Alternatively, as mentioned in a comment by John Bollinger, prefix the set commands with an @ (e.g. @set "JAVA_HOME_CONDA_BACKUP=%JAVA_HOME%"), or by prefixing the whole file with @echo off. The @ suppresses echoing on a command-by-command basis, whereas @echo off disables echoing altogether.