Using XML::XSH2, a wrapper around XML::LibXML:

open file.xml ;
for //tm mv text() replace . ;
save :b ;
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Note: Does not work if <tm>'s can be nested.

Update: If <tm> can contain something more than just a text, you have to use the more general "unwrap" method:

for //tm xmv node() replace . ;
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Tested with

<tm>a<b>c</b>c<?run test?><!-- comment --></tm>
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