docs/user_guide/build_tb: Removed ADI_TB_DIR export variable requirement

Signed-off-by: Istvan-Zsolt Szekely <istvan.szekely@analog.com>

Author: IstvanZsSzekely

docs/user_guide/build_tb.rst

@@ -153,7 +153,6 @@ located. Two variables must be exported, which specify the target directories:
    :showuser:
 
    $export ADI_HDL_DIR=<path to cloned HDL directory>
-   $export ADI_TB_DIR=<path to cloned Testbenches directory>
 
 Building a testbench
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